Re: [PATCH v3 05/15] KVM: x86: Add support for KVM userfault exits

[James Houghton <jthoughton@google.com>]

On Tue, Jun 17, 2025 at 9:24 PM James Houghton <jthoughton@google.com> wrote:
>
> Only a few changes are needed to support KVM userfault exits on x86:
>
> 1. Adjust kvm_mmu_hugepage_adjust() to force pages to be mapped at 4K
>    while KVM_MEM_USERFAULT is enabled.
> 2. Return -EFAULT when kvm_do_userfault() when it reports that the page
>    is userfault. (Upon failure to read from the bitmap,
>    kvm_do_userfault() will return true without setting up a memory fault
>    exit, so we'll return a bare -EFAULT).
>
> For hugepage recovery, the behavior when disabling KVM_MEM_USERFAULT
> should match the behavior when disabling KVM_MEM_LOG_DIRTY_PAGES; make
> changes to kvm_mmu_slot_apply_flags() to recover hugepages when
> KVM_MEM_USERFAULT is disabled.
>
> Signed-off-by: James Houghton <jthoughton@google.com>
> Signed-off-by: Sean Christopherson <seanjc@google.com>

This patch fails to remove the WARN in recover_huge_pages_range(). The
diff below will be applied to the next version of this patch, whenever it
comes.

This WARN can be hit by enabling KVM_MEM_LOG_DIRTY_PAGES and
KVM_MEM_USERFAULT, then disabling KVM_MEM_USERFAULT.

I've been having offline discussions with Sean about this series; I'm
waiting for him to rework the KVM_GENERIC_PAGE_FAULT bits. I'll dedicate
some more time to the QEMU side of things too.

Thanks.

diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
index 7f3d7229b2c1f..2d83ddb233a9a 100644
--- a/arch/x86/kvm/mmu/tdp_mmu.c
+++ b/arch/x86/kvm/mmu/tdp_mmu.c
@@ -1779,7 +1779,7 @@ static void recover_huge_pages_range(struct kvm *kvm,
 	u64 huge_spte;
 	int r;
 
-	if (WARN_ON_ONCE(kvm_slot_dirty_track_enabled(slot)))
+	if (kvm_slot_dirty_track_enabled(slot))
 		return;
 
 	rcu_read_lock();